To understand the differences in the body's responses to exercise, you need to familiarize yourself with the stages of menstrual cycles. The follicular phase, which extends from the end of your period to ovulation(day 7 through 14 or so of a 28-day cycle), is the point at which your body gets ready to release an egg and grows the endometrium, the lining of the uterus, to prepare for pregnancy. This phase tends to coincide with higher estrogen levels. Ovulation occurs in the middle of your cycle, from day 14 to around day 21. The luteal phase, which occurs after the body releases the egg during ovulation, lasts from ovulation until your period (days 21 through 28-ish), increases progesterone and then decreases both hormones suddenly in preparation for bleeding. "Several studies have looked at differences in responses to strength training in the follicular phase, versus training in the luteal phase," says Hardy. "Some research has found that strength training during the follicular phase resulted in higher increases in muscle strength compared to training in the luteal phase. If you start paying attention to your cycle phases, you may find your strength training pays off the most in your follicular phase."